The AR family of weapons and their derivatives include indirect gas operated versions, have been in use by the military and civilian population for many years. Firearms based on the AR family, including the AR-10, the AR-15 and the M16, are the primary weapon of choice for military units in the United States and abroad. It also is a rifle platform of choice among civilian sport marksmen and hunters.
Currently, this firearm platform is configured only for beltless cartridges. Examples of such cartridges include the 7.62×51 mm NATO cartridge, the 308 WIN cartridge, and other smaller caliber cartridges. This feature of the AR platform limits the choice of ammunition available for rifles in this family. In particular, it limits the shooting distance and barrier penetration possible with this platform. Currently, rifles in this family have a maximum range of about 800 meters, and a kinetic energy of less than about 2,400 ft lbs, exemplified by the 7.62×51 mm NATO cartridge. Rifles in this platform also tolerate a maximum operation pressure in the range of about 50,000 psi.
A belted cartridge is a cartridge having a shell casing with a pronounced “belt” around its base that continues 2-4 mm past the extractor groove and allows for proper headspacing in more powerful and larger cartridges. The addition of the belt to the casing prevents over-insertion into the chamber. Over-insertion can lead to catastrophic failure of the gun when fired with excessive headspace.
The use of larger and more powerful cartridges than those that can be used in the current AR platform are gaining popularity both in the military and in law enforcement, as well as among target shooters and big-game hunters. As one example, the 0.300 Winchester Magnum, a belted cartridge competent to travel over 1,800 meters with a kinetic energy in the range of about 3,800-4,000 ft lbs, is the most popular 0.30 caliber magnum with American hunters. It also is being adopted by law enforcement and certain branches of the military for long-range sniping and marksmanship. The desire for larger, more powerful (and belted) cartridges is at odds with the current AR platform.
There remains a need, therefore, for a modified AR platform competent to accept belted cartridges, particularly larger cartridges that safely, accurately and reliably extend the range and barrier penetration of the existing platform, while tolerating the enhanced pressures (in the range of at least about 65,000 psi) that these cartridges generate when firing. Particularly advantageous would be the ability to provide such a modified platform without substantially altering the characteristic features of the platform that have made it a weapon of choice